The present invention relates to a method and apparatus for producing a continuous molten tube of a film-forming material and, more paraticularly to such a method and apparatus in which the tube is formed by extrusion in an improved fashion, permitting fine adjustment of film thickness.
A number of different techniques are used to form extrudable, synthetic resinous thermoplastic compositions into film material. The two major methods for the preparation of plastic film by extrusion are tube extrusion and slot die extrusion. In the tube extrusion process, the heat plastified material, extruded in the form of a tube, is stretched over a suitable support, such as a mandrel or, alternatively, a trapped gas bubble. The gas bubble is formed between the extrusion die and nip or pinch rolls spaced from the die.
Two variations of the slot extrusion process are commonly employed. In the first variation, chill roll casting, the extrudate from a slot or sheeting die, is deposited on a cooled roll which is rotated at a surface speed in excess of the extrusion rate. The thickness of the resultant film is thus controlled by the thickness of the extrude, the relative speed of extrusion, and the surface speed of the chill roll. A second variation of slot extrusion is tentering, in which a sheet is extruded, grasped at the edges by mechanical supports, and stretched both in the direction of extrusion and in the transverse direction.
Although maximum control of the extrudate, including stretching rate, is obtained with the tentering process, this process is mechanically complex and requires that a relatively large portion of extrude along the edges be discarded as scrap. Chill roll casting, on the other hand, is mechanically simple but imposes severe restrictions on the molecular orientation which may be introduced into the film, especially in the direction transverse to the direction of extrusion. Tube extrusion offers substantial advantages in the rate of production possible and in the reduction of scrap production. In typical prior art tube extrusion from a generally circular slot defined by die lips in the extrusion die. The die lips extend generally parallel to the longitudinal axis of the tube. The construction of such a conventional tubular die is such that adjusting the thickness of this circular slot to equalize the flow rate around the die requires an adjustment in an outer concentric ring of the die with respect to an inner ring. It will be appreciated, therefore, that the fine adjustment which is available on a flexible lip slot extrusion die is not possible with such a conventional tubular die, since independent adjustment around the die is not possible.
Accordingly, it is seen that there is a need for an improved extrusion process and apparatus for producing a continuous molten tube of a film-forming material in which flow rates around the die may be equalized and in which other extrusion properties are facilitated.